Ambiguity interval limitation is a classic issue for distance measurement techniques.
For distance measurement based on phase shift, the issue has been addressed by using different frequencies of amplitude modulation: a lower frequency for resolving ambiguity, and a higher frequency for accuracy.
In time-of-flight distance measurement, the delay between emission time of an emitted laser pulse and detection time of a return pulse allows for distance calculation. When the measurement rate is high (e.g., 1 MHz), the time between emitted pulses is short, (e.g., 1 μs), so that the return pulse from a first emitted pulse can return after a second emitted pulse or even a third emitted pulse has been sent. This ambiguity makes it unclear whether a return pulse corresponds to the first, second or third emitted pulse.
If distance is measured between the emission time of a given emitted pulsed and the detection time of a return pulse resulting from a former emitted pulse, then the measured distance is diminished by a multiple of the ambiguity interval limit (e.g., −150 m for 1 μs of light traveling duration). So for example, a building at 193 m would be incorrectly measured at 43 m. Incorrectly measured distances result in three-dimensional point measurements which interfere with other, correct measurements (e.g., when measurements are displayed as a point cloud), with no easy way to filter or separate the incorrect measurements from the correct measurements.
In aerial scanning, where the variation of measured distances is small relative to the measurement range, the issue can be addressed by time windowing. See, for example, International Patent Publication WO 2008/107129 A1 dated 12 Sep. 2008, and U.S. Pat. No. 8,212,998 B2 dated 3 Jul. 2012. The system can expect a return pulse within a measurement window: a reasonable assumption for airborne LIDAR where the distance to be measured is always between two limits given by the flight altitude and the possible ground variations. This method is not suitable for terrestrial laser scanning where measured distances vary from the minimum to the maximum possible measurement range.
One solution for terrestrial scanning is to apply pulse signatures to the emitted pulses. See for example International Patent Publication WO 2009/039875 dated 2 Apr. 2009, and U.S. Pat. No. 8,149,391 dated 3 Apr. 2012. Applying pulse signatures makes it possible to distinguish pulses from one another. Recognizing the order of emitted pulses in a sequence enables the return pulses to be sorted in correct order. Pulses can be “signed” in different ways, such as by emitting a doublet of pulses for each distance measurement with the time (and thus the distance) between the two pulses of the doublet varying according to a defined sequence. The signature is recognized as the time between a pair of received pulses. The pulse signature approach has the disadvantage of implementation complexities, such as generating doublets, differentiating doublets from one another, and recognizing the signature from a noisy return doublet.
Additional ranging techniques are described in Patent Application Publication US 2010/0128248 A1 dated 27 May 2010, Patent Application Publication US 2011/0038442 A1 dated 17 Feb. 2011, European Patent Application Publication EP 2 469 297 A1 dated 27 Jun. 2012, Patent Application Publication US 2012/0257186 dated 11 Oct. 2013, and European Patent Application Publication EP 1 413 896 A2 dated 28 Apr. 2004.
Simple and effective techniques to address the ambiguity interval limitation are desired, especially techniques useful for terrestrial scanning.